1. Field of the Invention
The present invention relates to image forming apparatuses using electrophotographic systems and electrostatic systems, and particularly to image forming apparatuses such as copying machines, printers, and facsimile apparatuses.
2. Related Background Art
In image forming apparatuses, such as copying machines, printers, and facsimile apparatuses, using as developer two-component developer with main components of toner and carrier, toner is spent and a developer density in a developing apparatus gradually decreases as image formation proceeds.
Accordingly, the developing apparatus is equipped with a toner supplying apparatus for supplying toner, and toner in the toner supplying apparatus is occasionally supplied to the developing apparatus based on an output of a sensor for sensing the density of developer in the developing apparatus, such that the density of developer in the developing apparatus can be kept constant.
Further, a toner image (a test patch) for detection of an image density is formed on a photosensitive drum, and the image density is read by an image density detecting sensor. The thus-read value of the image density is compared with a reference value, and the toner supplying apparatus is accordingly driven. Toner is thus supplied such that the output of the image density detecting sensor can be always maintained at a constant value, thereby controlling the image density to obtain an appropriate image.
However, even when the image density is controlled by a bias using the developer density detecting sensor and the image density sensor as discussed above, fogging is likely to occur due to excessive supply of toner if the developer density is forcedly recovered at the time of recovery operation subsequent to detection of an anomalous condition of the developer density. Even in the event that no fogging appears, variation in the image density is liable to occur.